Minimum metal content (MMC) mine detectors having a search head and circuitry for detecting buried non-metallic and metallic land mines are well known. For example, U.S. Pat. No. 4,016,486 in the name of Pecori assigned to the United States of America by the Secretary of the Army, discloses such circuitry. An MMC detector includes electronic circuitry to assist a human operator to determine the nature of a solid obstruction encountered below the surface of the ground. Typically, metals and rocks are distinguished from one another. Metals are potential land mines. A prodder capable of distinguishing threats, from non-threats reduces stress and fatigue of a human operator and speeds up the process of clearing an area of buried land mines. The search head is typically a UHF balanced bridge detector which is unbalanced by passing the search head over a soil area which has a dielectric constant different from the background. Such a condition exists when passing over a mine.
Currently, instrumented prodders are known having ultrasonic means in the form of an ultrasonic transducer at or near the probe tip that are used for characterization of buried obstructions; this device can be used in conjunction with an MMC detector wherein the MMC detector first detects the ground indicating the vicinity of a land mine, and, wherein the instrumented prodder is used to probe the earth in the vicinity of the suspected land mine, the location of which may have been isolated using the MMC detector.
A Hand held prodder having a probe in the form of an elongate, preferably non-magnetic rod including a gripping handle disposed at one end is currently known. The design of the probe is based partially upon a Split Hopkinson Pressure Bar (SHPB) apparatus. In the apparatus a compression wave or high frequency elastic mechanical pulse is delivered via a to a sample wherein a portion of the wave is reflected. Mechanical impedance is a characteristic of a material. An incident wave launched at a material will be reflected and /or transmitted from or through the material, respectively, in dependence upon the characteristics of the material. The effect of mechanical impedance on a SHPB apparatus in three instances is described hereafter:
Firstly and obviously, if the mechanical impedance of a sample under test is the same as that of an incident bar in the SHPB, there will be no reflection as the sample will be displaced in a same manner as the bar itself as the compression wave is delivered. The displacement of the end of the bar is directly proportional to the strain measured (.epsilon.).
Secondly when the mechanical impedance of a sample is considerably greater than that of the bar, a sample's mechanical impedance tends toward being infinite and substantially the entire wave is reflected.
In a third instance when the mechanical impedance is zero, in the absence of a sample, the reflected wave is tensile but of equal magnitude to the incident wave. The phase of the wave is shifted by .pi. and the net stress is zero; the relative displacement at the bar end equals twice that for the first instance (2.epsilon.).
In a SHPB device, once the relative displacement of the bars is known, the displacement of the sample is ascertained. Taking into account Young's Modulus (E) and the displacement of the bar, the imposed stress can be calculated, wherein the force applied is equal to the product of the stress and the cross-sectional area of the bar.
Since the loading on the sample becomes equal after a short time, the analysis may be somewhat simplified. Strain results may be used for only the incident bar; or alternatively, the striker bar may be directed to impact directly on the sample, and the transmitter bar alone may be used to define the sample characteristics.
It is has been found that plastics, minerals and metals may be discerned from one another by using this approach.
It has been further found that a hand held prodder having a rod modified to be analogous to the incident bar of a SHPB may be used to detect or discern metal, plastic and rocks.
The prodder rod is provided with one or more piezoelectric transducers capable of generating an acoustic wave into the rod and for detecting reflected waves from an object contacting the end of the rod. Conveniently, signal processing means are coupled to the transducers and are provided for analyzing the detected reflected waves for determining the characteristics of the object; more especially distinguishing landmines from inert rocks. The signal processor establishes measurements of the frequency-time-amplitude characteristic of the object. The reflected waves are compared with known characteristic signatures of a plurality of materials to attempt to ascertain a match within predetermined limits.
Although instrumented prodders of this type may function satisfactorily in many instances, they suffer from a problem related to the fact that acoustic coupling at the obstruction is a function of the applied force to the probe end.
Preferably, enough force will be applied to the probe end such that characterization of the obstruction can occur without causing detonation; and, preferably, a relatively consistent force will be applied to the probe end such that an accurate determination as to the character of the buried obstruction can be made. However if too little force is applied at the probe end, a poor reading may result and a mine in the vicinity of the probe may go undetected. Too much force applied at the probe end in the vicinity of a land mine may inadvertently detonate the mine.
It is therefore an object of the invention to provide a method and device, which will overcome the aforementioned problems, related to too much force, too little force, or a varying force being applied to the probe end while in use.
It is a further object of the invention to provide an instrumented prodder for detection of land mines and the like that includes a force sensor for sensing a force such as pressure applied to an end thereof.